The complex, iron nitrosyl carbonyl [Fe(NO).sub.2 (CO).sub.2 ], is known to dimerize butadiene to produce vinyl cyclohexene (VCH). It is believed that the catalytic activity is due to the Fe(NO).sub.2 fragment. The methods known to prepare this complex require the reduction of [Fe(NO.sub.2).sub.2 Cl].sub.2 or the reaction of nitric oxide (NO) on a mixture of iron and FeCl.sub.3 in an electrolytic cell and in the presence of butadiene. The iron nitrosyl catalyst is believed to be produced in situ and used therein to obtain the dimerization. Reference to these methods is found in U.S. Pat. No. 4,238,301. Another patent, U.S. Pat. No. 4,234,454, discloses the preparation of various metal nitrosyl catalytic solutions by employing the combination of manganese, zinc or tin together with iron, cobalt or nickel nitrosyl halides to produce the respective metal nitrosyls in a system for dimerizing various conjugated dienes. An earlier patent, U.S. Pat. No. 3,510,533, discloses the dimerization of conjugated dienes with n-allyldinitrosyliron complexes and a method for their preparation. Several methods are given involving the reduction of a .mu.,.mu.'-dihalotetranitrosyldiiron.
The methods of reduction known to the art have numerous disadvantages. Some of these are: (1) the reduction of the metal nitrosyl compound is required to be in the presence of the butadiene, (2) many of the reducing agents themselves will polymerize the butadiene, (3) contaminants are introduced by the reducing agents and (4) catalyst solutions cannot be stored and expected to retain their activity. The dimerized products of the process of the invention may be useful in themselves or as intermediates to other products. Thus, for example, 4-VCH can be (1) chlorinated to make an insecticide, (2) oxidized to make benzoic acid, (3) reacted with hydrogen sulfide in the presence of acid-type catalysts to make sulfur-containing resins and (4) reacted with hydrogen sulfide in the presence of ultraviolet light to make a .beta.-mercaptoethyl cyclohexane.